The present invention relates to transmitters used in the process control industry to measure levels of materials in storage vessels, such as tanks. More particularly, the present invention relates to a microwave level transmitter that is capable of detecting material interfaces that are separated by a small distance.
Microwave level transmitters are used in the process control industry to measure levels of materials contained in a tank by transmitting a microwave pulse into the materials contained in the tank. The microwave pulse reflects off the contents of the tank and a return profile of the tank or waveform is generated. The waveform represents the amplitude of the reflections of the microwave pulse as a function of time. Peaks in the waveform represent received wave pulses corresponding to portions of the microwave pulse that were reflected off discontinuities within the tank. These discontinuities can include various material interfaces such as an air-material interface at the surface of the material in the tank, a liquid-liquid interface, such as a layer of oil on water, a liquid-solid interface, and a solid-solid interface. The location or levels of these material interfaces can be established using common Time Domain Reflectometry (TDR) principles once the corresponding time locations of the received wave pulses or peaks in the waveform are established relative to a reference time location.
Detection of the time location of the received wave pulses generally includes analyzing the waveform for peaks which exceed a threshold value. Typically, a single received wave pulse is detected by locating starting and ending points along the waveform where the waveform crosses a threshold value. This method will fail to detect multiple received wave pulses corresponding to multiple material interfaces, however, when the received wave pulses overlap to the extent that the starting and ending points encompass more than one received wave pulse. This overlap can be due to the close proximity of the material interfaces. The portion of the waveform that includes such overlapped pulses is defined as a twin peak pulse. The typical method can only detect the time location of received wave pulses that correspond to material interfaces that are sufficiently distant such that the waveform does not contain a twin peak pulse.
One possible method of detecting overlapping received radar wave pulses of a twin peak pulse is disclosed in U.S. Pat. No. 5,969,666 to Burger et al. (Burger). The method disclosed in Burger first locates a maximum value of a waveform or echo profile and searches backwards in time to locate various maximum and minimum slopes. The maximum and minimum slopes are used to distinguish the overlapping received wave pulses of the twin peak pulse (described in Burger as a double blip). In addition to being computationally intensive, the method disclosed in Burger always presumes that the waveform contains a twin peak pulse. As a result, even when twin peak pulse is not present in the waveform, the method disclosed in Burger will unnecessarily perform computations in search of overlapping received radar wave pulses.
A method and apparatus for detecting the presence of a twin peak pulse in a microwave level transmitter is provided. A received waveform is determined to contain a twin peak pulse when both a first peak point relating to first received wave pulse and a valley are detected in the waveform. In one aspect, a microwave level transmitter includes an interface detection module that is configured to use the above method to detect the existence of a twin peak pulse in the waveform.